On 21 Apr 2006 22:28:02 -0700, wrote:

Ignoramus12493 wrote:
Thanks for yesterday' suggestions regarding 3 phase transformers.

I have a hobart cybertig welder. It has a "six phase rectifier" shown
in this schematic:

http://igor.chudov.com/tmp/firing/6-phase.jpg

I received a three phase SCR control/firing system made by PCTI, that
seems to be extremely easy to hook up.

By the way, PCTI loves children and is highly recommended, they are a
wonderful company. The best of the best.

I want to convert this Cyber Tig to both TIG/Stick (CC) as well as MIG
(CV) mode. This way I could both tig and mig with it.

Here's its wiring diagram:

http://igor.chudov.com/tmp/firing/pcti-firing.jpg

My issue is that the PCTI schematic refers to a "three phase bridge"
and my welder has a "six phase" rectifier where all SCR's "point" to
one DC bus, and where another side is connected to an interphase
transformer. Very different schematics.

So, I have maybe one and maybe two possibilities:

1) Rewire the transformer. It is made of six secondary windings, two
independent ones on each phase. Two leads from two transformers on
every phase are tied into the interphase transformer.

The ones that are tied together I could disconnect them from
interphase transformer, parallel each pair, and then connect them so
that I have three leads instead of six. (I am not 100% sure how to
connect them, but I think that it is easy. I have an idea).

Then simply put together a new rectifier bridge wired as PCTI
suggests, using new SCR half bridges which I already have in my pile.

2) Maybe there is a possibility of wiring in the PCTI board so that it
works just fine controlling the six SCRs that make up the current "six
phase" rectifier.

I spoke to their engineer yesterday (did not give him the schematics)
and he was kind of vaguely positive about it, I asked if ony K2, K4
and K6 were used for synchronization and he said yes.

What I am thinking here is that I would wire all six existing Hobart
SCRs to the PCTI board, it would monitor voltage on K2, K4, K6, and
then adjust phase angles of firings of all six SCRs based on desired
current/voltage vs. actual. And that it would not matter that the SCRs
1, 3, and 5 are wired the opposite of the PCTI sample schematic.

Note that the SCRs that I have now are pointing with their gates to
the DC+ bus, which makes it impossible for K2, K4, and K6 to
meaningfully monitor voltage/phase. So, what I think I can do here is
turn all SCRs UPSIDE DOWN, so that K2, K4, and K6 could be connected
to 3 phase voltage.

The side effect of this is that the negatve bus will become positive,
and vice versa, but that is no big deal as I could simply switch
contacts leading to commutator.

Is this making any sense? Is approach #2 possible?

thanks

i

In a 6 pulse bridge the diodes are in series and so you allways need to
fire them in pairs. The gate pulses consist of 2 pulses 60deg apart. In
your original rectifier they are fired sequentialy 1 at a time, so the
fireing pulse trains are not compatable. It is possible to seperate
them with some logic but that would mean making an extra board.


Thanks... Sounds like you are fully right.

i